add support for WebVTT cue settings

Samples:

https://www.revvel.io/api/view/caption/0PDDNbQLvmT/2kQWZbp1lBj/en

http://fusion.ddmcdn.com/subtitles/longform/2017-12/18/160000.011.01.001-146999.vtt

i will post two screenshots for current behavior and what is expected(only the placement, ignore the background color it's not declared in the subtitle).

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